1. Field of the Invention
The present invention relates to a film forming apparatus, a film forming method, a method for optimizing a rotational speed and a storage medium which are for forming a multi-layer film of a reaction product by repeating the cycle of supplying at least one reactive gas to a surface of a substrate, such as a semiconductor wafer.
2. Description of Related Art
As a film forming method for use in a semiconductor manufacturing process, a film forming process is known which involves a repetition of a processing cycle of supplying a first reactive gas to a surface of a semiconductor wafer (hereinafter also referred to simply as “wafer”) as a substrate in a vacuum atmosphere to adsorb the gas onto the wafer surface, and then switching the gas to a second reactive gas to form an atomic or molecular layer or layers by a reaction between the first and second gases. By repeating the cycle a number of times, a multi-layer film of the reaction product can be formed on the substrate. Such a process, e.g. so-called ALD (atomic layer deposition) or MLD (molecular layer deposition), is capable of controlling with high accuracy the thickness of a film by the number of processing cycles and is good in the quality of the in-plane uniformity of the film, and therefore can effectively deal with semiconductor devices which are becoming thinner.
Such a film forming method may preferably be used for the formation of a high-dielectric constant film e.g. as a gate insulating film. When a silicon nitride film, (SiN film), for example, is to be formed, an SiH4 gas or the like may be used as a first reactive gas (raw material gas), and NH3, activated N2 or the like may be used as a second reactive gas.
A one-by-one processing ALD method is being studied which uses a one-by-one processing film forming apparatus comprising a reaction container having a gas supply port at one end, and which supplies a reactive gas to a substrate from the side in one direction and discharges the reactive gas, which remains unreacted, and a reaction by-product from the reaction container in another direction.
Patent document 1 discloses a technique for forming a film by such an ALD method. The technique involves supplying a reaction species with a carrier gas to a substrate surface repeatedly in a pulsed manner.
A vertical CVD apparatus for forming a film by allowing a plurality of reactive gases, contributing to the formation of the film, to flow alternately is being studied as a batch-type ALD apparatus for carrying out the above-described film forming method. Patent document 2 discloses a vertical CVD apparatus for forming a film by such an ALD method. The CVD apparatus supplies two processing gases, contributing to the film formation, alternately into a vertical reaction chamber while discharging the gases, so that the gases are adsorbed onto a large number of substrates disposed in the reaction chamber and are reacted to form a film. The amounts of the reactive gases supplied are controlled by controlling the flow rate of each gas with a mass flow controller (MFC) provided in a gas supply pipe. The pressure in the reaction chamber is controlled by controlling the amount of exhaust gas through adjustment of the degree of opening of an exhaust valve provided in an exhaust pipe.
Generally in a CVD apparatus, a substrate is sometimes rotated in a reaction container in order to reduce the influence of asymmetry inherent in the apparatus on the substrate. For example, a substrate may be rotated in order to eliminate the influence of asymmetry of the position of an exhaust port relative to the substrate, the influence of a gas supply direction, etc. and to enhance the uniformity of the thickness of a film in the substrate. If the time for supply of a reactive gas is sufficiently long compared to the rotation period, the non-uniformity of the thickness of a film due to the influence of gas supply direction upon the formation of the film is not a significant problem.
In an ALD apparatus, however, a film forming process is carried out by repeating sequential processing cycles as described above. In an ALD process, a first reactant is introduced into a reaction chamber via a gas inlet or a manifold and forms a deposited layer on a substrate. An excess of the reactant gas is then exhausted from the reaction chamber in an exhaust step (see e.g. patent document 3). An inert purge gas is supplied from a gas inlet into the reaction chamber as necessary to discharge the remaining reactant. After the exhaust step, a second reactant is introduced into the reaction chamber. The second reactant introduced reacts with the deposited reactant to form an intended substrate layer. An excess of the reactant is then exhausted from the reaction chamber in an exhaust step. Layers may be added to the substrate surface by sequentially introducing and exhausting additional reactant gases into and from the reaction chamber. The time for supply of each reactive gas is tens of seconds at the longest and a dozen seconds at the shortest. A reactive gas is thus supplied for a shorter period of time in an ALD apparatus as compared to the conventional CVD apparatus.
The above-cited patent document 2, which describes the batch-type ALD apparatus, teaches in paragraph 0020 that NH3 as a second feed gas is supplied for 5 to 120 seconds. The patent document 2 also teaches in paragraph 0021 that a DCS (SiH2Cl2: dichlorosilane) gas as a first feed gas is supplied instantaneously by using a gas pool in a gas supply pipe.
In the conventional CVD method, whether it be ordinary pressure CVD or reduced pressure CVD, a raw material is supplied to a reactor while, at the same time, an exhaust gas and an unreacted raw material are exhausted from the reactor so that the pressure in the reactor is kept constant during the formation of a film. Under such conditions, the flux of the raw material (amount of the raw material that passes through a predetermined area per unit time), flowing onto a substrate surface, is considered approximately constant irrespective of positions on the substrate surface. Consider now the adhesion probability of the raw material (“adhesion probability” herein refers to the value obtained by diving that portion of the flux which actually makes a film by the total flux in a certain area). SiH4 and SiH2Cl2 (dichlorosilane), main Si film materials, have a low adhesion probability, whereas non-Si raw materials have a high adhesion probability. With increase in the adhesion probability, the raw material is more likely to be consumed (i.e. make a film) near a gas supply opening and a decreased amount of the raw material flux reaches an area of a substrate surface which is remote from the gas supply opening. A decrease in the raw material flux directly leads to a decrease in the thickness of a film. Thus, a high adhesion probability promotes the formation of a film on a substrate in an area near the gas supply opening and retards the film formation in an area remote from the gas supply opening.
Also in an ALD process, the adhesion probability affects the adsorption rate of a raw material vapor and the rate of a reaction between adsorbed raw materials and, when the time for supply of a reactive gas is short, can cause the problem of non-uniform film thickness in a substrate due to the influence of gas supply direction. In addition, the recent movement toward larger substrates makes the problem more serious because a gas supply nozzle must deal with a large substrate processing area.
Therefore, also in an ALD apparatus, a substrate is sometimes rotated in a reaction container in order to reduce the influence of asymmetry inherent in the apparatus on the substrate.
The time for supply of a gas is short in an ALD apparatus. Therefore, when a substrate is rotated, it is possible that the gas may be supplied to the substrate only in directions within a certain angular range as viewed from the substrate depending on the combination of the substrate rotation period and the timing of the supply of the gas, or even may be supplied only in the same direction when the substrate rotation period fully synchronizes with the timing of the supply of the gas, resulting in the formation of a non-uniform film on the substrate.
Some prior art documents address this problem. Patent document 4 discloses a technique of making a substrate rotation period non-synchronous with the timing of the supply of a gas while the gas is supplied a predetermined number of times.
Patent document 5 discloses an apparatus which, based on a mathematical expression, controls a substrate rotation period and the timing of the supply of a gas so that they are made non-synchronous while the gas is supplied a predetermined number of times.
Patent document 6 discloses an apparatus which, depending on the number of ALD processing cycles, makes the cycle period different from a substrate rotation period.
Patent document 7 discloses an apparatus which makes a peripheral position on a substrate, to which a gas is supplied in an ALD processing cycle, different from a peripheral position on the substrate to which a gas is supplied in the next processing cycle.
The techniques disclosed in the patent documents all control a substrate rotation period and the timing of the supply of a gas so that the gas is supplied to varying positions on a substrate during a predetermined number of ALD cycles. Such techniques, however, cannot exclude the possibility that the gas can be supplied to the substrate in the same direction under specific conditions.